Natural gas at the outlet of production wells is often associated with formation water containing dissolved salts such as, for example, sodium chloride, potassium chloride, calcium chloride, sodium bicarbonate. The natural gas is transported from the production site to a processing site by circulation in lines. In cases where the natural gas is water-saturated and at equilibrium with an aqueous phase, depending on the transportation conditions, hydrate plugs leading to production stop may form. To prevent such problems, a hydrate inhibitor such as glycol is injected into the gas-carrying lines. An aqueous solution containing between 60% and 90% by weight of glycol can be used. After transportation, a mixture consisting of formation water and of glycol is recovered, then processed in a glycol regeneration unit in order to reconcentrate the glycol, i.e. to remove the water. The regenerated glycol can again be injected into the natural gas carrying lines.
Glycol distillation systems for separating the glycol from a mixture comprising water and glycol are known to the man skilled in the art. In general, the systems of the prior art allow an aqueous solution containing between 70% and 90% by weight of glycol to be obtained.
However, regeneration of the glycol leads to concentrate the salts initially present in the formation water in the regenerated glycol. The presence of these salts is often the cause of operating problems linked with the accumulation of these salts on certain parts of the regeneration device. The parts of the regeneration device which are the most sensitive to this salt accumulation are often hot elements, for example the reboilers that equip distillation columns. Accumulation of these salts can lead to irreversible deterioration of these hot elements and induce glycol degradation.
There are membrane separation techniques for carrying out separations on glycol-based liquid compositions. The existing membrane separation techniques can be distinguished by the method of operation of these membranes, this method of operation being generally linked with the stresses or with the driving force applied on either side of the membrane interface.
American patent U.S. Pat. No. 5,505,855 describes a method using a reverse osmosis type membrane separation technique. This patent describes a method of cleaning a contaminated and substantially water-free glycol wherein said glycol is heated, pressurized and passed into a reverse osmosis membrane module to recover a cleaned glycol-based permeate and a retentate comprising the pollutants initially present in the contaminated glycol.
American patent U.S. Pat. No. 5,817,889 describes a method using a membrane separation technique wherein the stress applied on either side of the membrane interface is an electric potential gradient, in this case by means of a technique referred to as electrodialysis. More precisely, this patent describes a method of cleaning glycol solutions comprising salts, wherein an amount of water allowing to recover a hydrocarbon-based phase and a glycol and water based phase is added to the glycol solution, these two phases are separated, and the glycol and water based phase is subjected to a salt elimination treatment using an electrodialysis technique allowing separation of the cations and of the anions of these salts.
The glycol cleaning methods of the prior art using membrane separation techniques involve a certain number of drawbacks that can be linked with the type of membranes used or with their method of operation. For example, the reverse osmosis type membrane interfaces have very small pore sizes, typically below 10 Angstrom, which can cause fouling. These membrane types also require a working pressure often above 4 MPa. Similarly, the membranes used by means of an electric potential gradient, electrodialysis type membranes for example, are very sensitive to the nature of the feed to be treated in terms of selectivity and of fouling. Furthermore, this type of membrane is quite fragile and can readily undergo physical deterioration if the feed contains particles of abrasive nature. Besides, most of the membranes of the prior art are not suited for processing high feed flow rates.
The present invention proposes a method of regenerating an aqueous glycol solution allowing to remove the salts and part of the water while overcoming or limiting the aforementioned drawbacks.